Nucleic acids and proteins have a central role in life on earth today. These structures, composed of nucleotides and amino
acids, provide the catalysis, genetics and some of the structure for life. The genetic information in DNA is transcribed
to messenger RNA, and this information is then translated from RNA to protein in the ribosome using transfer RNA.
There are other polymers in contemporary living systems that probably did not have an important role in the first life on
Earth. For Example, carbohydrates, chains of sugars, are used today for energy storage and plant cell walls but are not
likely to have been present in the first life on Earth.
How many different biopolymers were assembled and relied upon for the first life? It would be easier to agree on which
molecules were needed if scientists could agree on a definition of life. Three possible definitions can be given which illustrate
the different views. A minimalist definition of life is that of a self-sufficient system maintained by replication with change
(mutation). This simplest version of life, version 1, could have been an assemblage of nucleic acids sustained by an
external source of nutrients (energy). It might have been bound to a mineral surface which acted as a kind of
organizational structure. Some scientists feel that this minimalistic life was unlikely, preferring version 2 where the first
life is surrounded by a semi-permeable barrier (membrane) which may have maintained the integrity of the system more
efficiently than having it bound to a mineral surface. A yet more complex model of the first life, version 3, requires the
presence of the molecular machinery (proteins) to metabolize the ingested nutrients so that they can be converted to the
essential monomers of the machinery of life instead of assuming that they were provided by an external source.
Which molecules were required to form the first life? Version 1 could require only RNA, since RNA can both store
genetic information and catalyze reactions. Genetic material is essential because genetic information was needed to
direct the preparation of proteins and the process of self-replication, even if proteins are not included in the first life on
Earth. RNA and a semi-permeable barrier would be all that is required for version 2. The presence of polypeptide
catalysts in version 2 of life may have been needed to catalyze the formation of the barrier surrounding this life. If the
barrier protecting version 2 life from the environment was a membrane composed of lipids like those in present day
membranes, it may have also required embedded proteins to control the migration of nutrients through the membrane.
(A lipid is a non-polar biological molecule that has little or no solubility in water. The lipids of the current day cell wall
are components of long chain fatty acids containing 14 - 20 carbon atoms bound to glycerol or a glycerol derivative.)
Version 3 of the first life would require proteins to catalyze metabolism since it is unlikely that RNA had the catalytic
capability to carry out all these transformations, although it could complete some of them. Version 2, the "middle of the road"
scenario for the first life, will be considered in this review where the prebiotic synthesis (spontaneous formation on the
primitive Earth) of RNA an some catalytic peptides will be required.
RNA and proteins are polymers made from repeating monomers. In RNA the individual monomer units are composed
of a phosphate group bound to a sugar which is in turn bound to one of the four bases: adenine, guanine, uracil or
cytosine. In contemporary life there are 20 amino acids in proteins that differ in the structure of the R group.
General Considerations of Polymer Synthesis
Formally, the synthesis of the polymers appears simple. When two monomer units link together, they release a
molecule of water. However, the formation of either proteins or RNA from their monomers is not energetically
favored. (In other words, in order to create proteins or RNA from a collection of loose monomer units, you must
input a minimum amount of outside energy in the form of chemical energy.) The bond in between individual units
in proteins is unstable in water as it is in RNA, so energy is required to link two monomers together in the presence
of water. Because of this phenomena, energy input was necessary to have made RNA and polypeptides on
primitive Earth.
Initial studies using thermal energy (heat) to drive the formation of polynucleotides and polypeptides from monomers
were only marginally successful even if they were carried out in the absence of water. Polymer formation in the
presence of water is a more plausible prebiotic scenario since it is likely that water was prevalent on the primitive
Earth. Therefore, the only way to prepare RNA or proteins in the presence of water is to supply the required
energy to "active" them (to change the structure by adding a reactive group) thus making polymer bond formation more
favorable. While current theories suggest that RNA or protein was involved in early life, scientists have yet to provide a
feasible explanation for how the individual activated monomers would have been formed on the early Earth.
Life on the Rocks and the Formation of the RNA World
The general consensus among origins of life scientists is that RNA was the most important molecule in early, but not
necessarily the first, life on Earth. This consensus is based on the following data:
RNA can store genetic information in its sequence of bases.
Single stranded RNA exhibits catalytic activity. RNA in the ribosome acts to catalyze protein synthesis in present day
cells.
Laboratory studies have demonstrated that RNAs with no catalytic activity can evolve to structures with various
catalytic functions.
As previously stated, it can be difficult to form long polymers in water. The RNA polymer strand is formed step by step, and it
is thought to be necessary to form many smaller pieces of RNA before one might generate long strands. For example, a
50 nucleotide strand of RNA cannot be formed without first having considerable numbers of strands with 2 to 49
nucleotides each. Large chains bind more strongly than shorter ones to minerals, and minerals increase the local
concentrations of RNAs so they are more likely to react.
The general conclusion from the available data in this area is that only polymers that do not decompose quickly when
water is abundant and which bind to minerals might grow in the presence of water.
The short RNA chains formed directly from monomers would not have been sufficient to initiate a system functioning only on
the activity of RNA molecules; this is why it is important to understand the use of the minerals (for example, structured clays)
in facilitating the growth of RNA molecules. It has been proposed that RNA containing more than 40 nucleotide units
would have been able to replicate well enough to maintain the core information content of their sequences. In addition, some
scientists believe that a 40 unit long strand of RNA is the minimum chain length required for RNA to catalyze the reactions of
other RNA molecules. Because of the catalytic potential or larger RNA molecules, the possibility of RNA growth using
mineral structure supports the hypothesis of the formation of a world based on the self-replicating characteristics of RNA.
Ribozymes2
Messenger RNA in eukaryotic organisms contains intervening sequences that are transcribed but are not part of a gene.
The initial mRNA transcript needs to be processed to remove these intervening sequences, called introns, before the
mRNA leaves the nucleus. The splicing together of the functional pieces of mRNA (the exons) is usually accomplished
with protein enzymes. In the 1980's special sequences of RNA were found to be "autocatalytic". In other words,
without the involvement of any other molecule, some RNAs have sequences within their introns that form loops or
other structures that can act as catalysts to remove introns in the strand that contain them. Usually such elimination of the introns
in RNA is performed by a protein enzyme, making this RNA-catalyzed reaction unnecessary. Some scientists believe
that there was a time when all reactions, such as forming RNA strands from monomers, were catalyzed by RNA. They
use these findings to support their theories. Because of most of the RNA-catalyzed splicing reactions which go on in cells
are unnecessary today, it is logical to believe that these behaviors are residual from another time when they were important.
These conclusions support the concept of the RNA world where both catalysis and information storage was done by RNA.
1Edited from Ferris, James P. (unpublished transcript) by E. Vrolyk, July 2000.
2Geoffrey Zubay. Origins of Life on the Earth and in the Cosmos. (San Francisco, Academic Press,
2000) 153-156.
